Welding together low and high carbon steels

ABSTRACT

A weld joins a thin overlay of low carbon steel to a base that contains high carbon steel, at least at its surface along which the weld is formed. The weld may be effected by fusion (melting) or by solid-state diffusion. With either it creates a heat affected zone (HAZ) in the base around the weld. The HAZ contains enough austenite, and perhaps bainite as well, to render the HAZ relatively ductile and also crack resistant. Adjacent to the weld the HAZ has a hardness that does not exceed 58 HRC. The weld may be produced with a high energy beam or with resistance welding equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/118,311, filed Apr. 29, 2005, and derives priority from and otherwiseclaims the benefit of that application.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

This invention relates in general to welding and, more particularly, toa process for welding high and low carbon steels together and to aweldment formed by the process.

If steel is heated above its upper critical temperature, which variesdepending on the amount of carbon in the steel, the steel assumes aphase known as austenite, which is a solid solution of iron and carbon.Should the steel undergo a rapid cooling, some of the austenite willtransform into martensite, which is extremely hard, while much of therest will remain as retained austenite, which is considerably softer andmore ductile, although other ductile constituents will usually bepresent as well. The amount of martensite that is formed depends to alarge measure on the amount of carbon dissolved in the austenite at thehigher temperature. High carbon steels contain harder martensite thanlow carbon steels. Being harder, high carbon martensite steel resistswear and deformation and is therefore useful in bearings, gears, andproducts for like applications. But that very same steel lacksductility, that is to say, it is brittle and tends to crack. Low carbon(ferite, hamite) steel, when quenched, contains low carbon martensiteand some Nonmartensitic Transformation Products (NMTP). The low carbonquenched steel is not as brittle, but then it does not resist wear anddeformation as well.

A case carburized part, such as a bearing race, possesses the beneficialcharacteristics of both high and low carbon steel. It has a ductile corethat can withstand shocks and a hard case that withstands deformationand wear.

Joining materials by welding can occur either by melting the materialsin the vicinity of the joint or by avoiding melting and creating asolid-state diffusion bond. If melting occurs during the weldingprocess, that process is called fusion welding. Consider the situationof fusion welding a component made from two steels. When solidificationbegins, crystals of austenite form on the unmelted surfaces and grow insize and quantity as solidification progresses. The carbon concentrationof the solid must be less than that of the liquid from which it formed,so excess carbon remains in the liquid. The liquid is enriched withcarbon. As the temperature continues to decrease, the melted zonebecomes mostly solid with a diminishing volume of liquid remainingaround the grain.

As the last liquid freezes to solid, that freshly formed solid materialshrinks in volume. The solid phase has a smaller volume. The shrinkagecreates residual stresses within the fresh solid, know as a melt zone.With continued cooling, most of the higher temperature austenite phasetransforms to ferrite, pearlite, bainite, and/or martensite dependingupon the carbon concentration and the cooling rate.

The cooling rate can be affected by a heat treatment immediately priorto welding that raises the temperatures of the two components. Thiselevated temperature causes the cooling rate of the solidified melt zoneto be retarded, thereby, allowing NMTP to be formed. The NMTP areresistant to cracking. Microstructures containing large volume fractionsof martensite are not resistant to cracking. Thus, the solidified andcooled melt zone does not crack in response to the shrinkage-inducedresidual stress because of the minimization of martensite formation.Preheating the components is a well-known practice, but preheating maynot be possible. Softening of the components, dimensional change,distortion, and/or undesirable scaling and tinting of the surface mayrender preheating undesirable.

Consider the situation of fusion welding steels without a preheat. Themass of metal in the components functions as a heat sink and rapidlycools the steel in the region of the weld—self quenching in effect—andas a consequence, the steel in the melt zone acquires a good measure ofmartensite. The solidified cooled melt zone formed during the welding oflow carbon steels without a preheat consists of the relatively soft lowcarbon martensite, some NMTP, and some retained austenite. The melt zonedoes not crack in response to the shrinkage-induced residual stresses.

Joining of two high carbon steels without a preheat presents a specialproblem for the welder. The solidified and cooled melt zone of highcarbon steel consists of relatively hard high carbon martensite and alesser amount of retained austenite. This brittle microstructure cracksin response to the shrinkage-induced residual stresses. Cracks withinthe melt zone caused by shrinkage-induced residual stresses are known as“solidification cracks” and “hot cracks”.

The foregoing has focused upon the melt zone. Now, consider thesituation of the heat affected zone (HAZ) adjacent to the melt zone. Theheat of welding raises its temperature above the upper criticaltemperature as well. As the HAZ cools, it is also subject toshrinkage-induced residual stresses. However, the material in the HAZremains cooler and stronger than the hotter melt zone. Cracking of theHAZ does not necessarily accompany solidification. If it occurs, it willoccur after a delay ranging from seconds to days.

Fusion welding of two low carbon steels results in a HAZ that is firstaustenitized and then cooled to form the crack-resistant compositemicrostructure containing low carbon martensite, NMTP, and some retainedaustenite. Fusion welding of two high carbon steels results in a heatedand cooled HAZ creating the crack-susceptible high carbon martensite andsome retained austenite. Thus, the HAZs of high carbon steels are proneto cracking.

Not all weldments contain fusion welds—the welds can be diffusion bonds.An example is friction stir welding. If there is no melting, a melt zoneformation and consequent shrinkage stresses fail to develop. Althoughthere is no melt zone, a HAZ is created on both sides of the joint. TheHAZ microstructure that develops is dependent upon cooling rate andcarbon concentration. In the absence of a preheat, the cooling rate willbe fast due to the self-quenching. The HAZ microstructure will alwayscontain martensite because of the rapid quench. The carbon concentrationis then the determining factor for HAZ microstructure. Solid statewelding of low carbon steels will create crack-resistantmicrostructures, and crack-susceptible microstructures will be producedin HAZs of high carbon steels.

When a welder fusion welds a low carbon steel to a high carbon steelwith no filler metal, a somewhat similar problem develops. Again a meltand a HAZ develop and undergo a self quench. The steel in the melt zonerepresents a mixture of high and low carbon steels, and as a consequencehas a carbon content intermediate that of the two steels. Usually, it isnot enough to create hard plate-type martensite, so the melt zoneremains relatively ductile. That much of the HAZ that lies with the lowcarbon steel is not sufficiently brittle to cause concern. However, theremainder of the HAZ, that is the portion that lies within the highcarbon steel, acquires considerable plate-type martensite and as aconsequence is hard and brittle and subject to cracking under stresses,both residual and applied. The problems with welding high carbon steel,either to more high carbon steel or to a low carbon steel, areparticularly acute with butt welding and fillet welding. But they appearin the laser lap seam welding and resistance welding as well.

Typically, the race of an antifriction bearing is formed from a casecarburized steel that has undergone a heat treatment to produce a hardsurface on the race, or else it is formed from a high carbon steel thatis through hardened in a heat treatment. But often a race must be fittedwith a shield or some other component, often a stamping formed from lowcarbon steel. Since welding is not a viable option under currenttechnology, the component is pressed over, snapped into or onto, or insome other way mechanically connected to the race. Welding would serveas a desirable alternative if practical. To be sure, procedures existfor lessening the deleterious results from welding high carbon steel tolow carbon steel. One is preheating. However, that softens both steels,perhaps more than desired. Another resides in applying temper pulses tothe weld after it is made. These, however, do not produce the desiredductility. Normally, they lower the hardness to no less than about 58HRC (Rockwell C) when preferably it should be less than 50 HRC. Thenagain there is traditional tempering, but it is a diffusion process thatrequires considerable time and still lowers the hardness of the highcarbon steel to only about 58 HRC.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of a weldment forming part ofthe present invention and made in accordance with the process of thepresent invention;

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a TTT diagram showing the manner in which the weldment, whenformed by a two stage process, is cooled;

FIG. 4 is a fragmentary sectional view showing high and low carbon steelelements before they are joined together by resistance projectionwelding; and

FIG. 5 is a sectional view of a resistance projection weld formed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, a weldment A (FIG. 1) includes twoelements in the form of a case carburized base 2 and an overlay 4 whichoverlies the base 2. In addition, the weldment A contains a lap seamweld 6 which joins the overlay 4 to the base 2, thus firmly attachingthose two elements together. The base 2 may take the form of a race foran antifriction bearing, whereas the overlay 4 could be a shield orperhaps a case forming part of a seal for excluding contaminants for theinterior of the bearing. At the weld 6, the base 2 possessesconsiderably greater thickness and mass than the overlay 4. The weld 6pierces the overlay 4, but not the base 2.

The base 2 (FIGS. 1 & 2) is an integral and unitary structure, but evenso it has a core 10 of low carbon steel and a case 12 of high carbonsteel. The core 10 constitutes by far the greatest mass of the component6, the case 12 being quite thin and simply diffusing into the core 10.Indeed, the case 12 extends over and envelopes the core 10. It thusprovides an exterior surface 14 on the base 2. The carbon content of thecore 10 by weight may range between 0.10% and 0.30%, with the remainderof the core 10 being essentially iron. The carbon content of the case 12at the surface 14 may range between 0.7% and 1.3% by weight. Havingundergone a heat treatment, the case 12 is quite hard. Indeed, itshardness may range between 58 and 64 HRC.

The overlay 4 is quite thin, it having thickness between about 0.030 and0.200 inches. It overlies the surface 14 on the hard case 12 of the base2, with the overlap equaling at least the width of the weld 6. Theoverlay 4 is formed from a low carbon steel containing between 0.09% and0.20% by weight carbon. More often than not, the overlay 4 is formedfrom steel sheet, no thicker than about 0.200 in. and preferably 0.059to 0.135 inches. It has a margin or edge 15.

The lap seam weld 6 pierces the overlay 4 and penetrates the case 12 onthe base 2, thus attaching the low carbon overlay 4 to the base 2 at thehigh carbon case 12 on the base 2. Within itself, the weld 6 iscontinuous and thus is considerably longer than it is wide. It is setinwardly from the edge 15. It creates (FIG. 2) a melt zone 16—actually,a zone of formerly molten metal—that extends through the overlay 4 andinto the case 12 and perhaps beyond into the core 10 of the base 2. Thesteel in the melt zone 16 is steel derived solely from the overlay 4 andbase 2. No filler metal is added. The weld 6 also creates a heataffected zone (HAZ) 18 that generally envelopes that much of melt zone16 that lies at least within the base 2, particularly the portion of thebase 2 that is occupied by the case 12. Its constituency is also steel,indeed that of steel with a carbon content of the case 12 or core 10 atwhatever depth one considers the HAZ 18. To be sure, the HAZ 18 may nothave the same hardness as that of the remainder of the case 12 or thecore 10, but at least its carbon content is the same.

When the base 2 is made of carburized steel (FIG. 2), the weld 6 isformed in a single pass with a high energy beam that is focused on theoverlay 4 at the location where the overlay 4 is to be attached to theunderlying base 2. Preferably, the beam is produced by a laser, althoughother high energy beams will suffice as well. The high energy beam ispowerful enough to melt the steel of the overlay 4 and the steel of theunderlying base 2 at the localized area where it is focused. It producesthe melt zone 16 and the HAZ 18.

The high energy beam elevates the temperature of the weldment A at thelocation where it is focused high enough to melt the low carbon steel ofthe overlay 4 and the high carbon steel of the case 10 on the base 2.The molten steel, upon cooling and solidifying, becomes the melt zone16. The high energy beam also elevates the temperature of thesurrounding steel in the case 12 above the upper critical temperaturefor the high carbon steel, but not hot enough to actually melt thatsteel. The heat transforms the surrounding steel into a solid solutionof austenite, and thus forms the HAZ 18. As the high energy beam moveson, the molten steel rapidly cools, its heat being dissipated quicklyinto the remaining regions of the base 2 and to a lesser measure intothe overlay 4. In effect, the melt zone 16 and the HAZ 18 undergo a selfquench.

To prevent a high hardness, mostly martensite structure from developingwithin the HAZ 18 of the carburized steel for the base 2, the power ofthe high-energy beam exceeds that which would be required just topenetrate into the high carbon steel case 12. Indeed, the power of thebeam is sufficient to raise the temperature of the HAZ 18 near theboundary interface between the melt zone 16 and the HAZ 18 to at least1750° F. The consequence of raising the temperature of the HAZ 18 to atleast this magnitude is to dissolve the available carbon in theaustenite. After the high energy beam advances, the austenite in the HAZ18 near the interface cools through the M_(s) temperature, its heatbeing dissipated into mass of the base 2. Owing to the increased carbondissolved in solution, the austenite in the HAZ 18 transforms into acomposite structure of largely martensite and retained austenite. Theaustenite volume fraction is now 30%-50%. In any event, the magnitude ofretained austenite in the HAZ 18 near the interface is significantlygreater than that existing outside the HAZ 18 in the high carbon case10. This leaves the HAZ 18 near the interface with a hardness notexceeding 56-58 HRC. The relatively high volume fraction of retainedaustenite is sufficiently ductile to prevent crack formation.

Deeper within the HAZ 18, farther from the area raised to greater than1750° F., the microstructure consists of a lesser amount of retainedaustenite and more martensite. But cracks do not form here for tworeasons: (1) the state of residual stress is considerably less away fromthe interface, and (2) the martensite contains lower carbon, lath-typemartensite that is free of microcracks and is less brittle.

Were the temperature of the melt zone 16 and the HAZ 18 to drop rapidlythrough the M_(s) temperature without dissolving the available carbon inthe austenite, martensite would begin to form, with the greatestconcentration being in the HAZ 18, because it possesses the highestcontent of carbon. Indeed, the martensite in the HAZ 18 would leave theHAZ 18 with a hardness as high as 63-64 HRC. Owing to the brittlenessand high residual stresses which accompany high hardness, fracturingwould likely occur along the boundary between the melt zone 16 and theHAZ 18.

Where the base 2 is homogenous in the sense that it is formed entirelyfrom high carbon steel and through hardened, the weld 6 is formed in twoor more passes. The first pass possesses considerably more intensitythan the subsequent pass or passes. The second pass serves to interruptthe cooling of the melt zone 16 and the HAZ 18. The second pass followsthe first pass in a matter of 3 to 5 seconds. That is to say, once thehigh energy beam in its advance leaves a specific point, the same oranother high energy beam in the second pass should visit the same pointin a matter of seconds. In any event, the temperature of the high carbonsteel within the HAZ 18 after the first pass should remain above theM_(s) temperature so that between the first and second passes at anypoint along the weld 6, the steel in the HAZ 18 remains as essentiallyaustenite. During the second pass, the temperature of the steel againrises, but not as high as during the first pass and less than theeutectoid temperature. Some, but not all of the steel, melts within themelt zone 16 but not in the HAZ 18. The steel in the HAZ 18 remains longenough above the M_(s) temperature to transform some of the austeniteinto bainite, which is relatively soft in comparison to martensite. Thehigh energy beam moves on, allowing the steel of the melt zone 16 andthe HAZ 18 to again cool, with the heat dissipating into the mass of thebase 2 and overlay 4. The steel cools below the M_(s) temperature wheresome of the austenite transforms into martensite, but not nearly as muchas if the steel were allowed to cool to ambient temperature after thefirst pass. Indeed, less austenite is present to make thetransformation. In any event, the steel of the high carbon HAZ 18, uponfurther cooling below the M_(s) temperature to ambient temperature,contains bainite, martensite, and some retained austenite, with thebainite amounting to at least 10% by volume fraction and preferablyabout 35%. The bainite and austenite are relatively soft and theirpresence along with the harder martensite leaves the HAZ 18 with ahardness at the interface of preferably 46-50 HRC and, in any event,less than 55 HRC. The melt zone 16 is even softer. As a consequence,residual stresses are reduced and fractures are less likely to developalong the boundary between the melt zone 16 and the HAZ 18. The initialcooling followed by the heating to interrupt the cooling followed by thesecond cooling may be demonstrated in a TTT diagram specific to thesteel of the case 12, particularly the steel at the surface 14 (FIG. 3).This procedure is also suitable for use with a case carburized base 2.

The overlay 4 may be attached to the base 2 with one or more resistanceprojection welds 26 (FIG. 5), each of which is free of fractures andotherwise characterized by relatively soft steel, not only in the weld26 itself, but in the region surrounding the weld 26. The base 2 can bemade of either carburized steel or high carbon through-hardened steel.The process can be applied to both. Each projection weld 26 may includea melt zone 28 which resides between and within the overlay 4 and thebase 2, but pierces neither. Avoidance of a melt zone 28 is sometimesdesired. Indeed, the melt zone 28 may be replaced by a pressure bondformed at an elevated temperature with the pressure and temperaturebeing such that solid state diffusion occurs. The lower temperature andheat of the process minimizes distortion. Even so, within the case 12 ofthe base 2 the weld 26 establishes a HAZ 30. In contrast to the lap seamweld 6 which is continuous or at least considerably longer than it iswide, the projection weld 26 is generally circular and confined to asmall spot.

To form the projection weld 26, the overlay 4 is placed over a die andstruck with a punch to impart an indentation to it on one face at thelocation where it is to be attached to the base 2 and a detent orprojection 32 on its opposite face (FIG. 4). Once the projection 32 isformed, the overlay 4 is placed over the base 2 in the location at whichit is to be secured, with the projection 32 being against the exteriorsurface 14 on the base 2. Indeed, the overlay 4 is forced firmly againstthe base 2 with an electrode 34 that bears against the overlay 4 aroundthe indentation that is behind the projection 32. With the projection 32bearing tightly against the surface 14 of the base 2, the electrode 34along with the overlay 4 against which it bears and base 2 are placedacross a source of electrical energy. It generates a current whichpasses through the projection 32 and heats the projection 32 high enoughto cause the overlay 4 to bond to the base 2 at the projection 32,either by fusion (melting) or solid state diffusion. If the magnitude ofthe current is such that it melts the projection 32, the projection 32disappears and the overlay 4 seats against the surface 14 on the base 2(FIG. 5). Not only does the current melt the projection 32, but it alsomelts the base 2 where the projection 32 bore against it. In short, thecurrent produces a melt zone 28 which lies within the overlay 4 and alsowithin the base 2. In addition, the current creates the HAZ 30 aroundand beneath that much of the molten zone 28 that exists within the base2. On the other hand, where the current produces a solid statediffusion, a HAZ 30 still develops.

If in the case of a fusion bond the melt zone 28 and HAZ 30 were allowedto cool to ambient temperature at this juncture, the melt zone 28 wouldacquire some martensite, but not enough to make it excessively hard andbrittle. After all, it possesses a carbon content somewhere between thelow carbon content of the overlay 4 and the high carbon content of thecase 12 on the base 2. But the HAZ 30, as a consequence of the rapiddissipation of heat into the mass of the base 2, undergoes a precipitousdrop in temperature. Much of the austenite would transform intomartensite if the HAZ 30 were allowed to cool below the M_(s)temperature, and this would leave the HAZ 30 extremely hard and brittle.The same holds true for a solid state diffusion.

But the projection weld 26, like the lap seam weld 6 for a high carbonthrough hardened base 2 is formed in a two-step process. The first stepcreates the melt zone 28 or at least a solid state diffusion bond andthe HAZ 30. The second step, consisting of one or more applications ofelectrical potentials reheats both. More specifically, before the steelin the HAZ 30 cools to the M_(s) temperature it is reheated at leastonce by placing the weld 26 across an electrical potential and directingcurrent through it. The one or more reheats elevates the temperature ofthe HAZ 30, but not enough to reach the eutectoid temperature. Indeed,these later applications of current produce enough heat and are longenough to transform some of the austenite in the HAZ 30 into bainite.Once the electrical potential is finally removed, the HAZ 30 and meltzone 28 or diffusion bond cool to ambient temperature, with most of thecooling occurring by the dissipation of heat into the mass of the base2—in effect, a self quench. As the temperature drops through the M_(s)temperature and approaches the M_(f) temperature, some of the austenitetransforms into martensite. However, the martensite is considerably lessthan it would have been had the cooling after the initial formation ofthe weld 26 not been interrupted.

Basically, the same two-step procedure that is used for joining theoverlay 4 to the base 2, whether the latter be case carburized orthrough hardened, may be used to join the overlay 4 to the base byresistance spot welding or by resistance seam welding. In both the weldoccurs in the absence of a projection 32.

Irrespective of whether the overlay 4 and base 2 are joined with the lapseam weld 6 or a resistance weld 26, the weld 6 or 26 that is formedcontains less martensite and more austenite and perhaps bainite thanthose produced by traditional procedures for welding low carbon steel tohigh carbon steel. And this holds true whether the base 2 be casecarburized or through hardened. The welding creates a bond between thelow carbon steel and the high carbon steel, and that bond may be afusion bond produced by melting the two steels or it may be a solidstate diffusion bond. Irrespective of the type of welding, the weldingelevates the temperature of the overlay 4 and base 2 and produces a heataffected zone (HAZ), and the HAZ, while containing martensite, also atthe weld contains enough austenite and may also contain bainite—all insufficient quantities to produce a hardness at that location notexceeding preferably 55 HRC and certainly not 58 HRC and to otherwiserender the HAZ crack resistant.

In the context of the welding processes, low carbon steel has a carboncontent of no more than about 0.30% by weight, whereas high carbon steelhas a carbon content of not less than about 0.60% by weight.

1. A process for welding first and second steel elements together, thefirst element being formed from a low carbon steel and the secondelement containing, at a surface thereon, a high carbon steel, saidprocess comprising: placing the two elements together, with the firstelement being against the surface of high carbon steel on the secondelement; heating the elements in a localized area where they aretogether to effect a bond between the steels of the two elements in thatarea, the heating further transforming the nearby high carbon steel inthe second element into a heat affected zone (HAZ) in which the highcarbon steel is austenitic; allowing the steel in the HAZ to cool yetremain austenitic and above the martensite start temperature; while thesteel in the HAZ remains austenitic, subjecting the localized area tofurther heating that elevates the temperature of the HAZ to less thanthe eutectoid temperature and to further cooling such that some of thesteel in the HAZ transforms from austenite to bainite; and thereafterallowing the steel in the HAZ to cool below the martensite starttemperature for the steel in the HAZ, whereby the steel in the HAZincludes bainite in addition to martensite, with the bainite being atleast 10% by volume fraction.
 2. The process according to claim 1wherein the steels in the localized area are allowed to cool bydissipating heat into the first and second elements beyond the localizedarea.
 3. The process according to claim 1 wherein heating the elementsoccurs in at least two successive stages, with the first heating stagerendering the steel in the HAZ austenitic; and wherein allowing theelements to cool occurs in at least two successive stages, with thefirst of the cooling stages being between the first and second heatingstages.
 4. The process according to claim 1 wherein the HAZ after thelast cooling stage contains about 35% bainite by volume fraction.
 5. Theprocess according to claim 1 wherein the second element has a carburizedcase and the surface of high carbon steel on the second element is onthe carburized case.
 6. The process according to claim 1 wherein heatingthe elements in the localized area melts the steel of the two elementsat the localized area to create a melt zone that is surrounded by theHAZ.
 7. The process according to claim 1 wherein the heating of theelements to melt the steels includes subjecting the two elements to ahigh energy beam where the elements are together.
 8. The processaccording to claim 1 wherein the heating of the elements to melt thesteels includes conducting an electrical current through the elementswhere the first element is against the second element.
 9. The processaccording to claim 1 wherein the carbon content of the low carbon steeldoes not exceed about 0.30% by weight and the carbon content of the highcarbon steel is at least 0.60% by weight; and wherein the hardness ofthe HAZ does not exceed 58 HRC.